The present invention relates to an impact tool having a reciprocating ram, an anvil positioned forward of the ram for receiving an impact force from the ram, a needle chisel holder for carrying a plurality of needle chisels longitudinally and movably, and for receiving the impact force from the ram through the anvil, and a cylinder that accommodates the ram, the anvil and the needle chisel holder being in coaxial alignment with one another. The impact tool is adapted to apply an impact to a workpiece by pressing the tips of the needle chisels against the surface of the workpiece.
A conventional impact tool of the type described above is provided with a grip member on the outer periphery of the cylinder, which an operator holds when carrying out an operation with the tool (for example, see Japanese Patent Application Post-Examination Publication No. Sho 41-5867). In using this conventional impact tool, however, vibrations generated in the tool are directly transmitted to the operator's hand through the grip member, which results in operator fatigue. Accordingly, impact tools designed to minimize vibrations transmitted to an operator's hand have been developed.
In one such type of impact tool, the cylinder is accommodated in a sleeve formed from an elastic material, and a grip is provided on the sleeve (for example, see Japanese Utility Model Application Post-Examination Publication No. Sho 61-7909). With this type of impact tool, however, it is difficult to accurately aim tips of needle chisels at a workpiece. Moreover, it is not possible to forcibly apply the tips of the needle chisels against the workpiece, which prevents a strong and effective impact effect from being obtained.
In another such type of impact tool, the cylinder is axially reciprocatably accommodated in a housing, with a pair of axially spaced annular spaces being formed between the outer peripheral surface of the cylinder and the inner peripheral surface of the housing. A coil spring is installed in each of the annular spaces. One end of the coil spring is engaged with the cylinder, and the other end of the coil spring is engaged with the housing, whereby axial vibrations transmitted from the cylinder to the housing are absorbed by the coil springs (for example, see Japanese Utility Model Application Post-Examination Publication No. Hei 2-7026 and Japanese Patent Application Unexamined Publication (KOKAI) No. Sho 60-180784). In using this tool, a pressing force applied by an operator to the housing is transmitted to the needle chisels through the coil springs to press the needle chisels against the workpiece during operation. However, due to the degree of rigidity that the coil springs are required to have, when the tool is used, even if when it is not in contact with the workpiece, strong vibrations are transmitted to the operator's hand.
In still another such type of impact tool, as in the case of the above-mentioned conventional impact tool, coil springs are respectively set in a pair of annular spaces. In addition, an annular elastic member is set in the front annular space; and the pair of coil springs has a small spring constant to thereby floatingly support the cylinder in the housing. The annular elastic member has a spring constant larger than that of the coil springs. When the needle chisels of the tool are pressed against a workpiece to perform a desired operation, a reaction force acting from the workpiece is therefore received by each of the annular elastic member and the coil springs (for example, see Japanese Utility Model Application Post-Examination Publication No. Sho 61-7908). In this tool, however, the annular space for providing the annular elastic member is limited in size. Therefore, the size of the annular elastic member cannot be increased, which prevents its use in an operation which requires the application of a strong force to a workpiece.